1. Field of the Invention
The present invention relates to an optical recording medium for optically recording and reproducing information, a substrate for the optical recording medium for use in the optical recording medium, and a method for preparing the substrate.
2. Related Background Art
Heretofore, an optical recording medium is constituted of a transparent substrate which transmits light for recording/reproduction such as a semiconductor laser beam, a recording layer disposed on the substrate, and if necessary, a protective layer on the recording layer. As a material for the substrate, there has been widely used a polycarbonate resin having excellent transparency and strength and low hygroscopicity. The polycarbonate resin referred to herein means a usual polycarbonate resin which has been heretofore industrially produced, for example, a bis(hydroxyphenyl)alkane series polycarbonate resin. The substrate for the optical recording medium (hereinafter abbreviated to "substrate") which comprises such a polycarbonate resin tends to cause deteriorations such as discoloration and the decrease of molecular weight by the irradiation of natural light (particularly its ultraviolet component), and therefore it is desired to prevent the photo-deterioration. For this prevention, a technique of adding a UV absorber to the substrate is known.
However, in the case that a usual UV absorber such as 2-(5-methyl-2-hydroxyphenyl)benzotriazole or 2- 2-hydroxy-3-(3,4,5,6-tetrahydrophthalimidomethyl)-5-methylphenyl!benzot riazole is used as the UV absorber and the optical recording medium made of the substrate containing such a UV absorber is allowed to stand for a long period of time under a high-temperature and high-humidity atmosphere, the UV absorber has an unpreferable influence on the recording layer sometimes. For example, when an organic coloring matter is used for the recording layer, the UV absorber deteriorates its reflectance, and when a photo-magnetic film is used as the recording layer, the UV absorber causes pitting corrosion in the film to lower a bit error rate inconveniently.